Nuclear Energy

Nearly all electrical energy is created by the heating of water in a boiler, creating steam or super-heated water, which is channelled through pipes to drive a turbine. By forcing the steam through a series of little propellers, or blades, a shaft can be rotated within a stator. If one of the two, the stator or the rotor, is fitted with powerful magnets, and the other with thousands of coils of wire, then electrical current is generated in the wire. This is then distributed through the mains network.

Where nuclear power differs from a fossil-fuel burning power generation plant is the source of the heat for the boiler. Fossil fuels are coal, oil and natural gas, so-called hydrocarbons, which are enthusiastic about breaking down and joining with oxygen. This 'enthusiasm' takes the form of an exothermic reaction called 'oxidation', in which hydrocarbons combust, releasing large amounts of chemical potential energy in the form of heat.

Nuclear reactors also produce large amounts of heat. They do this not by burning the fuel, but by accelerating a natural process, by which uranium atoms spontaneously decay into smaller atoms (decay products). In the process they release sub-atomic particles called neutrons, which can hit other uranium atoms, which in turn are caused to decay, releasing more neutrons, which hit other uranium atoms, and so on. The effect is a chain reaction, as more and more of the uranium atoms decay, releasing ever more neutrons. During the decay, a part of the mass of the nucleus is converted into pure energy, according to Einstein's famous equation: $E = mc^2$

If the uranium is in the form of a sphere, and there is enough of it, the chain reaction will go out of control, and a nuclear explosion results. To generate power, the rate of decay needs to be controlled so that the uranium does not go critical. In a nuclear power station, the uranium fuel is arranged in rods, separated by neutron-absorbing boron control rods, all in a heavy water moderator, so that the neutron generation rate is kept at a constant optimum. The heat that can be generated is therefore well-controlled, and electricity can be generated like in any other power station.

Greenhouse gas emissions (CO2) for nuclear power compare favourably with renewable energies, and are well below fossil fuels.

Sounds like a good system... So why are countries like Germany and Switzerland phasing out nuclear power?

Nuclear

P/V

Wind

Hydro

Coal

Oil

Methane

CO2 emissions (kt/TWh)

15

13

9

2

1,000

778

420

ERoEI

5 - 24)

3 - 7

16 - 25

10 - 270

Power generation Germany (2015) /%

14.1

5.9

13.3

3.0

43.6

0.8

8.8

Power generation Switzerland (2015) /%

37.9

1.2

0.15

.5

0.0

1.3

0.0

Cost (Germ.) ct/kWh

4 - 10

10 - 14

5 - 11

2 - 8

6 - 8

6 - 12

7.5 - 10

EU subsidies (billion €)

35

Renewables total = 30

Fossils total = 26

The Three Problems with Nuclear Energy

1. Nuclear Waste

We do not really have a guaranteed long-term system to store the used uranium for the tens of thousands of years necessary before it is sufficiently less dangerous (it will never be completely safe).

For issues concerning nuclear waste management see the article on 'Nuclear Waste' (click and follow link).

2. Nuclear Accidents

Plant safety has not had a clean record. There have been many accidents involving reactor operations and waste management. The radioactive materials from reactors can cause ionising radiation, which is dangerous for health and the environment. Radioactive fall-out plays very much into a primeval fear. There are also the problems of nuclear terrorism and proliferation of nuclear materials and technology, making the world a less safe place.

Fishermen protesting against the construction of the Kudankulam Nuclear Power Plant, March 2002

For accidents and proliferation issues see the article on 'Nuclear Safety' (click and follow link).

3. Economics

Energy Returned on Energy Invested, ERoEI: Nuclear = 5-15 (optimised plants up to 24), Photovoltaic (P/V) 3-7, Wind 16-25, Hydro = 10-270 (very dependent on location). Some reports claim the ERoEI for a new generation of nuclear power plant could be in the hundreds, but for now this remains theoretical.

Uranium supply and economic efficiency: far from being 'almost free', as was first touted in the 1950s, the costs of the nuclear industry still outweigh most other forms of electricity production. The high costs of decommissioning aged plants were typically grossly underestimated in original planning, if they were accounted for at all. As Germany is experiencing, there is no way to have the nuclear industry balance its books without the public purse picking up a large part of the tab.

Insight EU provides the following breakdown of energy subsidies in Europe in 2011: Nuclear = 35 billion Euro, Renewable energy = 30 billion Euro, Fossil fuels = 26 billion Euro, Efficiency measures = 15 billion euro. A large part of the subsidies for nuclear power is in the form of liability insurance, which guarantees the state pays for the costs following severe reactor incidents.

Fukushima nuclear power station, Japan, following the tsunami and resulting fire on March 11, 2011.

The German federal and Länder governments spent between 1956 and 2006 of the order of 50 billion euro on nuclear energy research and technology. This does not include decommissioning costs for installations, which amounted to 2.5 billion Euro, and 6.6 billion Euro for uranium mining redevelopment. Nuclear power stations in Germany in 2009 had an average operational availability of 74.2%.

Nuclear power is expensive and its waste product, depleted uranium fuel, must be stored for tens of thousands of years till it is 'safe'. Accidents can cause leaks of radiative material which leave large areas of land uninhabitable due to contamination, as well as spreading through the groundwater and sea, entering the human foodchain through fish.

Ticino, Switzerland, received a lot of radiation from the Chernobyl nuclear accident in 1986, from a contaminated raincloud. Japan, Russia, Ukraine, and the USA have all suffered serious nuclear accidents, releasing deadly radiation which will contaminate land, water, and food for thousands of years.

It is being phased out in Germany and Switzerland, but France still makes 75% of its electricity from nuclear power. Italy does not use nuclear power at all. Nuclear reactors in 2015 produced 13% of the world's electricity.

Reactor Types

There has been much investment in different types of reactor, whether to exploit different fuels, or the new generation of reactors, employing new technologies which were previously unknown or considered impracticable.

The nuclear power that has been used since the 1950s utilises the fission of uranium. The large uranium atoms, a mix of the 235 and 238 isotopes, is used as fuel. A mass of uranium fuel is allowed to enter a controlled chain reaction, and uranium atoms decay into smaller atoms, releasing neutrons and heat. The heat is created by some of the mass during the decay process being converted to energy, according to Einstein's $E = mc^2$, where $E$ is the energy released, $m$ is the mass deficit between the initial mass of uranium and the final products, and $c$ is the speed of light, $3.0 × 10^8 m/s$.

Pressurised Water Reactors

The most common type of reactor is the Pressurised Water Reactor, PWR, which uses enriched natural uranium oxide fuel (about 3-4% U-235 and the rest U-238), or MOX (mixed-oxide fuel).

Fast-Breeder Reactors

Fast breeder reactors (FBR) can make use of the less-fissile U-238 since they have a fast neutron spectrum.

Natural uranium is 99.3% U-238, and only 0.7% U-235. Since U-235 is fissile, and U-238 is not, uranium needs to be enriched, increasing the proportion of U-235 to about 3-4%. However, since there is still a large proportion of U-238 in the fuel pellets, whether intended or not, U-238 is transmuted to plutonium-239 due to neutron bombardment. A breeder reactor therefore 'breeds' fissile plutonium.

A Fast Breeder Reactor is a reactor design which attempts to 'breed' Pu-239 from U-238. It uses a liquid sodium salt coolant cycle. Although it would better use the bulk of the uranium fuel, the technology is being phased out due to technical difficulties in the breeding process.

The half-life of U-235 is 704 My. The half-life of U-238 is 4.468 billion years. This difference explains why U-238 has a natural abundance of 99.27%. U-235 has halved its quantity at least 8 times since its formation in the supernova which formed the solar system, 6 billion years ago. The quantity of U-238 has halved only once.

Uranium can be caused to decay at higher than natural rates in nuclear reactors. This releases huge amounts of heat which can be used to generate electricity. Uranium does not release pollution of the type fossil fuels do.

Waste and Spent fuel

Nuclear waste is either used fuel, or contaminated substances which have become radioactive due to exposure to ionising radiation.

When uranium-235 has undergone fission it produces pairs of decay products, including:

Spent fuel from reactors is stored in cooling tanks for as much as a decade, before it is moved to intermediary stockpiling

After an optimum quantity of U-235 has been used up in the fission process, the fuel can no longer generate enough heat to be used for power generation. The rods containing it must be removed from the reactor core. These can be recycled through reprocessing (re-enrichment), in plants such as the UK's Sellafield Plant, or must be stored till they have lost enough of their heat generating capacity, prior to longer-term storage. Long-term storage facilities are being investigated in many countries, but are not yet in operation. Worldwide there are about half a million tonnes of high-level nuclear waste awaiting a permanent storage solution.

Future Developments

CAESAR

The Clean And Environmentally Safe Advanced Reactor, CAESAR, is a design still in development, which proposes to utilise an initial quantity of LEU (low-enriched uranium) to start the reactor, which will then continue with only U-238 as fuel. Neutrons will be controlled by means of a steam moderator.

Thorium

Thorium is weakly radioactive and has seven naturally occurring isotopes, all of which are unstable to very varying degrees (half-lives vary from 25.5 hours for Th-231 to 14 billion years for Th-232!). Th-232 is by far the most abundant thorium isotope in the crust. It is 3 or 4 times as abundant as uranium.

There is a great deal of interest in utilising thorium as a fuel for nuclear reactors, especially in india, which has no uranium resources, but abundant thorium.

One atom of U-233 releases 197.9 MeV ($3.171 × 10−11$ J), or 19.09 TJ/mol = 81.95 TJ/kg, of energy during fission. The half-life of U-233 is 160,000 years, and alpha decays to Th-229.

Th-233 has a half-life of 22 minutes, and decays into Pa-233 (half-life = 27 days), which beta decays to U-233 (half-life = 160,000 years). U-233 usually fissions, releasing an alpha particle to form Th-229, when impacted by a neutron, but a portion of it keeps the neutron and becomes U-234. The overall capture-fission ratio is lower for U-233 than for U-235 and Pu-239, which means the chain reaction requires a higher neutron density to reach sustainable levels.

When U-233 undergoes fission, it emits neutrons, which can impact another thorium-232 nucleus, causing the decay to start again. The chain reaction would be self-sustaining at a critical mass and geometry. The cycle is similar to that in fast-breeder reactors, which produces highly-fissile Pu-239 from low-fissile U-238. However, thorium is more abundant than uranium, and offers a more sustainable supply, especially since the fissile U-233 it uses can be 'bred' from natural ore thorium. Most uranium reactors are 'burner' type, and its fuel is enriched natural uranium ore.

Thorium also has the advantage that it can be mixed with U-238, and therefore has no weapons-grade potential, which is not the case with U-235 enriched fuel. It also has a higher neutron yield, produces fewer long-lived transuranium elements, and makes better-performing reactor cores.

Thorium needs to be neutron irradiated before it can be used as a fuel, and this presents greater technological challenges, which is why it has not been adopted extensively to date.

Thorium-232 undergoes a complex series of transmutations to arrive at stable lead

One of the three cores in the US Shippingport Atomic Power Station was a 60MWe thorium breeder reactor. It operated from 1977 till 1982, when the experimental core was removed, to reveal that the quanitity of fissile material had actually increased by 1.4%, demonstrating the effectiveness of the breeding system. It used pellets made up of a combination of thorium dioxide and uranium-233 oxide.

Quote of the day...

Nonno's explanations always did a round-robin circuit of science - history - philosophy, then back to science. Always back to science. As if it were that that drove the mechanism of time, and not the other way round. And philosophy? Well, that just went along for the ride.